Fix for missing `glist_findrtext` in new Pd versions

[ben-wes]

this uses the mechanism we already had in place for the multichannel handling.

uses the new glist_getrtext if present and otherwise falls back to the previous glist_findrtext

tested with Pd vanilla for official 0.55-2 build and current master including the change.

[ben-wes]

ah, sorry - had tim's changes in my branch here already! will reset to master and force push ...

EDIT: done